The present invention relates to optical radiation detection apparatus; and more particularly, it relates to optical signal detection apparatus for use in diffuse infrared light wave communication of information.
Modern semiconductor technology has provided light source devices, such as light emitting diodes, which can be accurately controlled by a driving electrical signal to produce modulated light. There has also been provided by this technology photodetector devices, such as photodiodes and phototransistors, which generate an electrical signal in response to light incident upon a receiving surface thereof. The advent of such light source and photodetector devices has given rise to the use of light as a transmission medium for intelligence. In particular, infrared light wave communication is finding rapid development.
In communicating intelligence via a diffuse infrared communication link, infrared light, the intensity of which has been modulated, is emitted from a source omnidirectionally to diffuse throughout the bounding enclosure, typically the inside of a room. The emitted light diffuses by uncollimated spread of the beam and also by reflection off boundary surfaces, such as walls, floor, ceiling, furniture, etc.
The amplitude of the electrical signal generated by a photodetector device is a function of the total optical power incident on the receiving surface of the device. Incident optical power is itself a function of both the area of the receiving surface and the field of view of the device. Reasonably priced photodetector devices have a small receiving surface. Furthermore, the photodetector devices have a narrow field of view, with electrical signal amplitude falling off dramatically at wide angles of light incidence.
Thus, in diffuse optical channels, as encountered in infrared light wave communication, the total optical power received and converted into the electrical output signal is but a very small portion of the transmitted infrared light. To obtain a suitable electrical output signal for processing by an electronic receive circuit, an optical signal of high power must be emitted or the communication range be limited.
Of further consideration in infrared light wave communication is the introduction of noise in the optical channel by reason of exposure of the photodetector to ambient light. Ambient light sources, such as daylight and tungsten or fluorescent lamps, include components within their spectrum which generate shot noise in the photodetector. To compensate for ambient light shot noise, a higher optical power input to the photodetector is required.